Ptc thermistor of bati03,and other oxides

ABSTRACT

A PTC THERMISTOR COMPOSITION CONSISTING OF BATIO3, AL2O3, SIO2, TIO2, AND ONE OXIDE SELECTED FROM THE GROUP CONSISTING OF NB2O5, TA2O5, SB2O3, BI2O3, LA2O3, CEO2, GD2O3, SM2O3 AND Y2O3, AND A METHOD OF PRODUCING CERAMIC BODY FOR SAID PTC THERMISTOR COMPRISING MIXING THE CONSTITUENTS, CALCINING, PRESSING THE CALCINED MIXTURE INTO DISKS, FIRING THE DISKS AT 1240 TO 1400*C. FOR 0.5 TO 5 HOURS, AND COOLING THE FIRED DISKS AT A RATE LOWER THAN 150*C. PER HOUR.

June 22, 1971 Filed Oct. 22, 1968 RESISTIVITY INCREAS YOSHIHIRO MATSUO ET AL. 5

PTC THERMISTCR OF BaTi 0 AND OTHER OXIDES 2 Sheets-$hect 1 FIG. I

AC IOOV FIG 2 ON-OFF CYCLES Y. MATSUO E. KUROKAWA H. SASAKI S. HAYAKAWA.

Attorneys June 22, 1971 YOSHIHIRO MATSUO PTC THERMISTOR OF Ba'li 0 AND Filed Oct. 22, 1968 TEMPERATURE (C) 7(Furnoce cooling) 8(300 C/hf) 2 Sheets-Sheot 2 FIG. 3

9(150 C/hr) IOOO IO( 5O C/hr) 800 o A l l l l l TIME (HOUR) Y. MATSUO E. KUROKAWA H. SASAKI S. HAYAKAWA,

United States Patent 3,586,642 PTC THERMISTOR 0F BaTi0 AND OTHER OXIDES Yoshihiro Matsuo, Eisuke Kurokawa, Hiromu Sasaki, and

Shigeru Hayakawa, Osaka, Japan, assignors to Matsushita Electric Industrial Co., Ltd., Osaka, Japan Filed Oct. 22, 1968, Ser. No. 769,681 Claims priority, application Japan, May 29, 1968, 43/ 37,425 Int. Cl. H01b 1/06; C04b 33/00 U.S. Cl. 252--520 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of producing a ceramic thermistor having a positive temperature coefficient of electrical resistance, which will be hereinafter designated as PTC thermistor for convenience.

It is well known that the BaTiO ceramic shows a semiconductivity when being incorporated with a small amount of metal oxide such as oxide of a rare earth element: Y, Bi, Sb, Nb or Ta. The fired semiconductive ceramic body shows an anomalous PTC behavior in a specific temperature range, which will be hereinafter called PTC range. In both temperature sides lower and higher than said PTC range, the PTC thermistor has a negative temperature coefiicient of resistivity. The PTC range coincides with the temperature range where the dielectric constant of ferroelectric BaTiO varies with temperature in accordance with the Curie-Weiss law. The temperature at which the PTC behavior starts to appear coincides with the ferroelectric Curie-point at which the crystal structure transforms from tetragonal to cubic symmetry. The temperature at which the PTC behavior starts to appear will be hereinafter designated as PTC onset temperature. Therefore, a PTC onset temperature of the PTC thermistors can be shifted by a substitution of the constituent ions of the thermistor material compositions as well as in the case of ferroelectric BaTiO solid solutions.

The PTC thermistors have been recognized as promising components for electric devices. Possible applications thereof are for use as an element of a heating device with a self-current regulating action, over-heat protector, temperature controller, etc.

The conventional PTC thermistors tend to show an increase in the electrical resistance thereof with aging and the increase is accelerated by a high electric power loading. Such increase in the electrical resistance is most seriously observed in the lower temperature side than in the PTC range. Since the electrical resistance of the PTC thermistor in the lower temperature range determines the limit of electric current to be regulated, an increase in the electrical resistance is undesirable for practical application in a high electric power situation.

For these reasons, PTC thermistors have not served as heating device elements for regulating high electric power.

It is, therefore, an object of the present invention to provide a PTC thermistor characterized by high stability during the working thereof under high electric power.

It is another object of the present invention to provide a method of producing a PTC thermistor characterized by high stability under a high electric power test.

These objects of the present invention and the manner of their attainment will be apparent to the art-skilled upon consideration of the following description taken together with the accompanying drawings in which:

FIG. 1 is a block diagram showing an electrical circuit for high power aging tests.

FIG. 2 is a graph showing comparison of the stability of PTC thermistors according to prior art and according to the present invention.

FIG. 3 is a graph showing cooling curves of the ceramic elements for PTC thermistors.

The novel PTC thermistor according to the present invention is of a composition consisting essentially of 89.5 to 99.955 weight percent BaTiO 0.014 to 2.8 weight percent of A1 0 0.026 to 7.8 weight percent of SiO and 0.01 to 3.0 weight percent of Tl02, said A1 0 SiO and TiO being totally less than 10 weight percent, and 0.005 to 0.5 weight percent of an oxide selected from the group consisting Of Nb205, T8205, sbzoa, Blzog, b203, C602, Gdzog, 5111203 and Y203.

The conventional PTC thermistor is of a composition consisting of, for example, 99.8 weight percent of BaTiO and 0.2 weight percent of rare earth oxide or other dopant such as Bi O and Sb O It has been known that the conventional PTC thermistor with a single addition of La O is impaired in semiconductivity and PTC behavior when incorporated with a minor amount of A1 0 However, the novel compositions according to the present invention include A1 0 and show a good semiconductivity and a marked PTC behavior.

The fact that the compositions according to the present invention are not badly affected by an incorporation of A1 0 is advantageous for PTC thermistor manufacturing. Mixing and crushing steps in the usual ceramic manufacturing process employ a ball mill which is made of ceramic material including A1 0 The use of such ball mill results in a contamination by A1 0 which impairs the semiconductivity of the conventional PTC thermistor. However, the novel compositions according to the present invention are not impaired by the use of ball mill made of ceramic material including A1 0 A PTC thermistor according to the present invention has a high stability while working under high electric power. On the contrary, the conventional PTC thermistor is stable during working under an electric current less than ma. per cm. but is not sufliciently stable with an electric current as high as 500 ma. per cm.

The PTC thermistors according to the present invention are prepared in a manner similar to that of prior ceramic technique. The starting materials of a given composition are well mixed in a ball mill made of ceramic material and are pressed into disks at a pressure of 500 to 1000 kg. per cm. (kilograms per square centimeter). The pressed disks are fired in air at 1240 to 1400 C. for 0.5 to 5 hours. The fired disks are cooled to room temperature (15 to 30 C.) The ceramic disks are provided on both surfaces with ohmic aluminum electrodes prepared by the molten Al spraying method. Solders having a melting point higher than 350 C. are superposed on the aluminum electrodes by a metallizing method. Lead wires are attached to the electrode by soldering.

The resultant PTC thermistors are subjected to aging test under an electrical loading. The PTC thermistors for testing have a diameter of 30 mm. and a thickness of 3 mm. The PTC thermistor is connected in series with an Ni-Cr alloy resistor and the series circuit is supplied with AC 100 v. as shown in FIG. 1, where said PTC thermistor and alloy resistor are respectively designated as l and 2. The aging test is carried out by repeating the cycle of supplying the voltage on and off for 5 minutes and 3 minutes, respectively. Said Ni-Cr alloy resistor is adjusted in the resistance to limit the maximum current flowing through the PTC thermistor. The high electric power tests are carried out at a condition of maximum current density of 500 ma. per cm. This current density corresponds to approximately 7 amps with a disk type thermistor of 30 mm. in diameter. Such high current density is taken in view of the practical application of the PTC thermistor to a heating device.

Since the PTC thermistor has a low resistance in the lower temperature side than the PTC range, a large current, i.e. a rush current, flows through the circuit for a moment immediately after the AC 100 voltage is supplied. Accordingly, the PTC thermistor is self-heated by the rush current and reaches a temperature at which a thermal equilibrium is attained between the PTC thermistor and the ambit. Since the resistance of the PTC thermistor in the PTC range is much larger than that of an alloy resistor, the larger part of the supplying voltage is supplied to the PTC thermistor. Within the 3 minutes off-time, the PTC thermistor is cooled to room temperature. Before and after the aging test, the resistivity is measured and resistivity increasing rate is calculated. It is desirable to lower the increasing rate of resistivity as low as possible.

One aging test run has 5000 on-otf cycles of electric loadings.

In FIG. 2, the curve 3 is for the conventional PTC thermistor of a composition of BaTiO with 0.2 weight percent of Nb O and curve 4 is for the novel PTC thermistor of a composition of BaTiO with 0.225 weight percent of A1 1.9 weight percent of SiO 0.375 weight percent of TiO and 0.05 weight percent Nb O Both thermistors are prepared in a manner similar to that set forth above. They are fired at 1350 C. for 2 hours in air and cooled to room temperature at a cooling rate defined by curve 7 of FIG. 3. It is clear from FIG. 2 that the novel composition is superior to the conventional composition in respect of stability in the high electric power test.

According to the present invention, the cooling rate has a large effect on stability in the high electric power test. It is desirable that the fired ceramic body be cooled from the firing temperature to room temperature at a cooling rate less than 150 C. per hour. As a practical matter, an advantageous procedure is that the fired ceramic body is cooled at a cooling rate of 50 to 150 C. per hour in the temperature range from the firing temperature to approximately 400 C.

The following description will explain examples of the novel cooling rate. A PTC thermistor of a composition similar to that of curve 4 of FIG. 2 is fired at 1350 C. for 2 hours in air and cooled at a rate defined by curves 9 and 10 of FIG. 3. The results for the high power tests are shown in FIG. 2 as curves 5 and6. It will be clear on comparison of curve 4 with curves 5 and 6 that the novel cooling rate results in a high stability in high power tests.

A PTC thermistor is required to shift the PTC onset temperature depending on practical applications thereof.

Said PTC thermistor has a lower PTC onset temperature without impairing the semiconductivity thereof by a partial replacement of Ba with Sr. The larger the amount of replaced Ba, the lower the PTC onset temperature. Operable amount of replaced Ba is less than 40 atomic percent.

Said PTC thermistor composition has a higher PTC onset temperature without impairing the semiconductivity when the Ba atoms in an amount less than 30 atomic percent are partially replaced by an equivalent atomic percent of Pb. The higher the amount of the replaced Ba, the higher the PTC onset temperature.

The PTC onset temperature of said PTC thermistor is lowered without impairing the semiconductivity by substituting Sn, in an amount less than 30 atomic percent, for an equivalent atomic percent of Ti in said BaTiOg.

The substitution of Zr in an amount less than 20 atomic percent for an equivalent amount of Ti in said BaTiO also lowers the PTC onset temperature without impairing the semiconductivity.

Both substitutions of Pb for Ba and of Sn for Ti in the novel compositions of the present invention produce a PTC thermistor having a higher stability in the high electric power test. Advantageous substitution amounts are 1 to 30 atomic percent of Sn for Ti and 1 to 20 atomic percent of Pb for Ba in said BaTiO respectively. Both substitutions cause the PTC onset temperature of the resultant PTC thermistor to shift to the lower temperature side when the Sn substitution amount in atomic percent is larger than 0.4 times the Pb substitution amount in atomic percent. On the contrary, the PTC onset temperature of the both-substituted PTC thermistors shifts to the higher temperature side when the Sn substitution amount in atomic percent is smaller than 0.4 times the Pb substitution amount in atomic percent. When the Sn substitution amount in atomic percent is 0.4 times of the Pb substitution amount in atomic percent, the resultant PTC thermistor shows no shift of the PTC onset temperature.

In all cases of both substitutions, the resultant PTC thermistors show a higher stability in the high electric power test in accordance with the present invention.

EXAMPLE For the preparation of the PTC thermistor compositions listed in Table I, mixtures of BaCO TiO A1 0 SiO and one oxide selected from the group consisting of Nb205, T3205, Blzoa, Sb O 113203, C602, Gd Og, 8111203 and Y O were well mixed by a wet ball mill, calcined, and pressed at a pressure of 700 kg. per cm. into disks. The pressed disks were fired at various temperatures for various periods of time as shown in Table II. The fired disks were cooled from the firing temperature to 400 C. at a cooling rate defined by curve 7, 8, 9 or 10 of FIG. 3 and afterward the disks were allowed to cool to room temperature. The cooled disks were in a size of 30 mm. diameter and 3 mm. thickness and were provided, at both surfaces, with an ohmic Al electrode by the molten Al spraying method. Two lead wires were attached to the Al electrodes by using solder having a melting point of 350 C. The resultant disks were measured with respect to the PTC characteristics as shown in Table II and also subjected to the high electric power test in a manner hereinbefore described. Table II also shows the results of the high electric power tests.

It is clear from Table II that the PTC thermistors of compositions according to the present invention are superior to conventional PTC thermistors in stability in high power test. Table II also shows that the novel cooling rate according to the present invention has a marked effect to improve the stability in the high power test.

A n T I TABLE I. Continued Additives (weight percent) Additives (weight percent) Sample No. Principal composition A1 S102 T102 Nb205 Sample No. Principal composition A1 02 S103 T102 Nb O 0 0 0 0. 2 0 0 3 0. 2 M 0. 7 1,3 0,5 0, 5 n 'U a 0. 7 1.3 0.5 0.005 0,7 1,3 5 ()1 6 BaT o.1Sno.1Oa 0.5 1. 5 0. 5 0.1 0. 7 1. 3 0. 5 0. 1 00.; 01.3 0.5 0.5 32 3 01 026 0. 01 0. 1 0.14 0. 26 o. 7 a 0. 7 1. 3 0 5 0. 5 0,175 2.15 0,175 .111 10 8 0.9 0.1 s 0. 7 1.3 0 5 0.1 2. 8 5. 2 2.0 0.1 7.2 5 0. 1 C602 7. 0 0. 1 0. 225 1. 9 0. 375 0. 05 a i a 0. 7 1. 3 0. s 0. 05 O. 225 1. 9 0. 375 0. 1 8.; 1.2 0. 5 0. l GdzOa 1. 0. 5 0. 1 0.7 1.3 0,5 04 80. Ti0s- 0.7 1.3 0.5 0.05 0. 7 1. 3 0, 5 0,1 1 Bi ms o.05 0.911SI1 .02Oa 0. 7 1. 5 0.5 0.1 0. 7 1. 3 0.5 0.1 g msggo05%010521150283. 1. g 0. g 1 SmaO a 00.15 0.05 0.15 1 0.05 3 0- M B210,\;Pb .1Ti sSn O 1. 0 1. 5 o. 5 0. 2 32 Ba i a 0. 7 1. 3 0. 5 0. 05 23 BaTiOa 0.7 1.3 0.5 0.005 2Q Y2 a Sba 33 Bao.95Pbo.05T10a 0. 7 2. 0 O. 5 0. 5

24 Same as above 0. 7 1.3 0.5 0. 5

TABLE II Column Increasing rate of rcsistivity (after Firing condition Resistivity 5,000 on-ofi Resistivity PTC onset at PTO on- FTC of Cooling cycle aging Sample Temp. Time at C. temp. set, temp. resistivity rate test No. C.) (111.) (ohm-cm.) C.) (ohm-cm.) (ohm-cm.) C 0.111.) (percent) FC 40 1 1,350 2 1, 000 120 050 13 300 40 150 FC 35 2 1, 380 2 500 120 450 13 150 50 35 FC 40 3 1,320 2 10 110 9, 000 12 300 35 150 35 FC 30 4 1, 320 2 800 110 700 15 300 20 150 8 FC 25 5 1, 320 2 30 110 28 15 300 10 150 8 FC 25 6 1 1, 320 2 5, 000 110 5, 000 10 300 15 150 8 FC 40 7 1, 380 2 500 120 450 10 300 35 150 30 FC 25 8 1 300 4 50 115 14 300 20 150 10 FC 25 9 1, 300 1 40 110 35 10 300 20 150 15 FC 30 10 1, 260 0. 5 100 105 95 9 15g 10 5 FC 30 11 1, 240 0. 5 70 105 70 9 300 15 150 10 F0 35 12 1. 1, 220 0. 5 10 100 10 5 300 150 20 FC 25 13 1, 350 2 100 150 95 14 150 10 7 FC 25 14 1, 260 1 300 210 300 5 150 10 50 7 FC 25 15 1, 320 2 35 30 10 300 15 150 8 FC 15 16 1, 340 2 1, 000 10 1, 000 10 150 5 50 5 F0 25 17 1, 340 2 40 70 40 10 300 15 150 8 FC 25 18 1, 340 2 10 20 150 10 300 10 150 7 FC 30 19 1, 340 2 70 8 300 20 150 12 FC 20 20 1,280 0. 75 90 10 300 8 5 TABLE II.-Contin'ued Column Increasing rate of resistivity (after Firing condition Resistivity PTO onset Resistivity PTO of Cooling cycle aging Sample at 25 C. temp. at PTO onresistivity rate test 0. Temp. Time (ohm-em.) C.) set, temp. (ohm-cm.) C.hr.) (percent) No'rIL-FO =Furnace cooling.

What is claimed is:

1. A PTC thermistor composition consisting essentially of 89.5 to 99.955 percent by weight of BaTiO 0.014 to 2.8 percent by weight of A1 0 0.026 to 7.8 percent by weight of SiO and 0.0-1 to 3 percent by weight of TiO said A1 0 SiO- and Ti0 being less than 10 percent by weight in all, and 0.005 to 0.5 percent by weight of one oxide selected from the group consisting of Nb O Ta O Sb O3, B1203, 1.13203, C602, Gd203, 811120 and Y2O\3- 2. A PT C thermistor composition according to claim 1, wherein Ba is replaced by an amount of Sr less than 40 atomic percent.

3. A PTC thermistor composition according to claim 1, wherein Ba is replaced by an amount of -Pb less than 30 atomic percent.

4. A PTC thermistor composition according to claim 1, wherein Ti is replaced by an amount of Sn less than 30 atomic percent.

References Cited UNITED STATES PATENTS 3,359,133 12/1967 Smyly 10639 3,373,120 3/1968 Nitta et a1. 252520 DOUGLAS J. DRUMMOND, Primary Examiner US. Cl. X.R. 10639; 252-521 

